1. Field of the Invention
The present invention relates to a gas analyzing apparatus, and particularly to the gas analyzing apparatus that emits and receives measurement light into sample gas flowing in a pipe so as to analyze concentration of a predetermined component.
2. Description of the Related Art
Flue gas exhausted from a boiler burning coal or heavy oil contains components such as sulfur oxide (SOx), nitrogen oxide (NOx), carbon dioxide (CO2), carbon monoxide (CO), and the like.
As a gas analyzing apparatus for analyzing content of each component contained in the gas, for example, there is an apparatus in which a probe is disposed in a gas flow path in the pipe so as to cross the same, measurement light emitted from a light source toward the gas is reflected by a reflector disposed at the tip of the probe, and hence concentration of components of the sample gas is analyzed based on information of the reflected measurement light (as shown in U.S. Pat. No. 5,781,306, for example).
FIG. 14 is a cross-sectional view schematically illustrating probes used for a conventional gas analyzing apparatus.
A probe A illustrated in FIG. 14 includes a probe tube B having a hollow tube shape in which the measurement light passes through. The probe tube B is attached to a pipe side wall D so that the probe tube B crosses the gas flow path in a flue C.
The pipe side wall D has an attachment portion E for attaching the probe A. The probe A is attached to the attachment portion E via a flange F.
In the proximal end portion of the probe A, there are disposed a light emission portion G for emitting measurement light into the probe tube B and a light receiving portion H for receiving reflection light. In the distal end portion of the same, there is disposed a reflector I for reflecting the measurement light from the light emission portion G to the light receiving portion H.
In the gas analyzing apparatus using the probe A as described above, gas in the flue C is led into the probe tube B, the measurement light emitted from the light emission portion G and reflected by the reflector I is received by the light receiving portion G. Thus, it is possible to analyze each component in the gas based on characteristics of the measurement light.
When the sample gas flowing in the flue C is led into the probe tube B, if the sample gas reaches an optical system member such as the light emission portion G, the light receiving portion H, or the reflector I via the probe tube B, the optical system member is exposed to the sample gas at high temperature and may be damaged by dust pollution or corrosion.
Therefore, in order to prevent the optical system members from being exposed to the high temperature sample gas, a purge gas feed portion J is disposed on the probe A, and purge gas is fed between a measurement region of the probe tube B and the light emission portion G as well as the light receiving portion H.
In addition, it is possible to adopt a structure in which the purge gas is fed into the distal end portion of the probe tube B through a purge gas feed tube (not shown) disposed in the probe tube B so that the reflector I can be prevented from being exposed to the sample gas.
In this structure of the probe A, there may be formed a gap K communicating directly to the flue C between the probe tube B and the attachment portion E.
In a case where the gas flowing in the flue C is flue gas, the temperature thereof is 100 to 400 degrees centigrade, and the gas flow rate is 5 to 25 m/sec. Therefore, if a part of the gas flowing in the flue C flows into the gap K, it is cooled in the gap by the purge gas led into the probe tube B at the outside air temperature. As a result, there occurs a difference of gas temperature between the upstream side in the vicinity of the gap and the downstream side.
If unevenness of ambient gas temperature occurs in this way, the temperature of the probe tube B becomes uneven, and particularly in a probe of the optical system having the hollow space in which the measurement light passes through, thermal lens effect phenomenon may occur.
Thus, there may occur a problem that the measurement optical axis fluctuates or is shifted due to an influence of the thermal lens effect phenomenon, and there is a problem that the light receiving state in the light receiving portion becomes unstable so that the measurement accuracy is deteriorated.